I think I am right to say that the causative mechanism between the two phenomena has not yet been uncovered .

Rather than ask what it might be ,can I ask what  main theories have been discounted?

 

Does all ongoing  research into this area require some kind of input from  quantum theory  or are there any purely classical theories that are also being pursued ?(I hoped I have  not mangled the terminology with my use of the term  "theories")

Relativity theory is considered a classical theory.  Some unification attempts to make it a quantum theory.

I came across this  recently

https://phys.org/news/2017-09-entanglement-inevitable-feature-reality.html#jCp

"Entanglement is an inevitable feature of reality"

 

where they seem to be working backwards from classical behaviour to put restrictions on any possible quantum theory that might be  possible..

I see no specific mention of spacetime curvature in that article(which does not go into what it says is a very complicated proof in the paper) .....

 

So not sure if it is relevant to the OP.

The article is irrelevant.  Try this to get some idea.

https://en.wikipedia.org/wiki/Quantum_gravity